rockchip: px30: Fix hard dependency to DEBUG_UART_BOARD_INIT

rockchip: px30: Fix hard dependency to DEBUG_UART_BOARD_INIT

Because DEBUG_UART_BOARD_INIT depends on DEBUG_UART, hard dependency
to DEBUG_UART_BOARD_INIT in ROCKCHIP_PX30 can cause warnings if
DEBUG_UART is disabled.
The DEBUG_UART_BOARD_INIT is already implied by ARCH_ROCKCHIP entry.
Remove hard dependency from ROCKCHIP_PX30, so that it will be
consistent with other rockchip boards.

Signed-off-by: Lukasz Czechowski <lukasz.czechowski@thaumatec.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>